When the call came in to the windowless bunker on High Street, the Red Line dispatcher swiveled around to the supervisor on the platform behind him.

“I have an emergency here,” he said, according to an official account.

In an instant, the two men were side by side, staring up at a two-story wall of video screens and real-time maps.

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An unattended train — a moving blip of LED numbers and a potential disaster — was rumbling toward Quincy Adams Station. Four miles down the line, another train, its operator unaware, was stopped at Wollaston, letting passengers on and off.

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The men knew they had 10 minutes, maybe, to stop the runaway train, clear the line, and avoid the kind of collision nobody wanted to picture — train cars crumpled and derailed, commuters endangered, millions of dollars in damage. Cutting the power to the entire Braintree branch would mean stranding other trains just as rush hour was beginning.

John Tlumacki/Globe Staff

Inside the MBTA’s Operations control room in downtown Boston where a dispatch team was able to bring the runaway train to a stop, MBTA Chief Operating Officer Jeff Gonnevillez explains what happened.

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They immediately radioed the train at Wollaston and the others ahead of it on the 8.8-mile Braintree branch to run express, making no stops, and set in motion an improvised plan to try to bring the runaway Red Line train — six cars and 420 feet of aluminum carrying about 50 passengers — to a safe halt.

Even in quiet moments, the MBTA’s Operations Control Center looks like a Hollywood set — think “War Games” or “Apollo 13” — but now the darkened room was alive with crackling drama.

The supervisor and the dispatcher had 48 years of MBTA experience between them, but neither had game-planned for this. Nobody had. Of the millions of radio and phone calls that have come in to 45 High St. over half a century, not one had ever carried a report of a runaway train tearing down the tracks of one of Greater Boston’s subway lines.

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Just a minute earlier, the same dispatcher had fielded a routine radio call from the operator of this very Red Line train as he tried to leave Braintree, the first station on the branch.

The motorman was having a signal problem. He knew it was safe to proceed, but a display on his dash and a traffic light-style signal alongside the track was telling him otherwise, just another infrastructure glitch in an aging transit system.

Using a radio that looks like an old-fashioned telephone handset, that train operator had called from his cab at about 6:08 a.m., to ask for permission to climb down and flip the manual bypass switch underneath the cab, near the side of the train. That would allow him to move the train forward despite the signal-system failure.

From a desk in the control center — adorned only with a “Feeling Stressed??” card carrying the number for the MBTA’s employee-relaxation line — the dispatcher granted the request.

Now, a minute later, an inspector on the platform at Braintree was calling to say that the train operator — his leg injured when the train grazed him — had just climbed up from the tracks and tumbled toward his booth. The train had taken off on its own.

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Later, investigators would learn that the control lever inside the train’s cab had been held in place with a cord and rotated in a direction that caused the train to accelerate after the motorman flipped the signal-bypass switch, according to a person familiar with the investigation.

The men between them had 48 years at the T under their belts, but neither had planned for this.

In that moment, though, the dispatcher and his supervisor raced not to determine the cause of the runaway train but to prevent disaster, according to MBTA chief operating officer Jeffrey Gonneville and chief transportation officer Todd Johnson.

Speaking to reporters at the control center on Friday afternoon, Gonneville and Johnson recounted the tick-tock drama that had unfolded the morning before in the same room.

They praised the dispatcher and supervisor — already gone for the day — but declined to name them or make them available, citing the ongoing investigation. They identified the control-center employees only as a supervisor who was hired in 1982 and a dispatcher who joined the T in 2000.

Responding immediately to the emergency, the dispatcher on Thursday morning radioed the train that was stopped at Wollaston and the others ahead of it on the inbound tracks of the Braintree branch and ordered them to run express in an effort to clear that section of the Red Line.

The next challenge was to stop the runaway train. If they cut the power to the third rail, that train — maybe going 25 miles an hour, maybe faster — would lose propulsion and eventually roll to a distant stop. But if they cut power at the wrong time, they could also risk stranding the other trains still on the Braintree branch — mooring scores of commuters trying to make it to work, or possibly worse.

As a backstop, the two men called for workers from the MBTA’s Cabot Yard, on the edge of Dorchester and South Boston, to run out to the tracks nearby and lay down “portable trip stops,” mechanical arms that can catch a passing train and stop it.

As those workers in the field scrambled to lay the stops near JFK and again alongside Cabot Yard, the men in the control center waited for the last train ahead to clear the final section of the Braintree branch and reach the JFK/UMass station. That is where the Braintree and Ashmont branches merge and head underground into downtown Boston.

About six minutes after the emergency call first came in, the two men watched that next train ahead clear JFK. They cut off power to the Braintree branch. Out on the Red Line, the MBTA’s first driverless train rolled to a stop just past North Quincy Station, more than 5 miles down the line from where it took off — disaster averted, a few minutes and a couple of miles to spare.